Examples of a semiconductor memory device (hereinafter, simply referred to as a memory device) include a DRAM and an SRAM, which are categorized as a volatile memory; a mask ROM, an EPROM, an EEPROM, a flash memory, and a ferroelectric memory, which are categorized as a non-volatile memory; and the like. Most of these memories including single crystal semiconductor substrates are already put into practical use. Among the above semiconductor memories, flash memories are widely marketed, which are mainly used for mobile storage media such as USB memories and memory cards. The reason of this is that flash memories are resistant to physical impact and can be conveniently used because they are non-volatile memories which can repeatedly write and delete data and can store data without being supplied with power.
As types of flash memories, there are NAND flash memories in which a plurality of memory cells is connected in series and NOR flash memories in which a plurality of memory cells is arranged in matrix. Any of these flash memories has a transistor which functions as a memory element in each memory cell. Further, the transistor which functions as a memory element has an electrode for accumulating electric charge, which is called a floating gate, between a gate electrode and a semiconductor film serving as an active layer. The accumulation of electric charge in the floating gate enables storage of data.
Patent Documents 1 and 2 describe a thin film transistor including a floating gate which is formed over a glass substrate.